1. Field of the Invention
The present invention relates to an inversion phenomenon preventing circuit of a video recorder/reproducer in which a video signal is stored and reproduced through frequency modulation, and more particularly to a circuit for recovering the absence of zero crossing in an FM signal reproduced from a storage medium in the video recorder/reproducer.
2. Prior Art
FIG. 15 is a block diagram for illustrating a reproduction system of a conventional video signal storage/reproduction apparatus. A very small FM signal, reproduced from a magnetic tape 29 by means of a video head 30, is amplified by an amplifier 31 and is corrected frequency response thereof by an RF equalizer 32. The FM signal from the equalizer 32 is amplitude limited by a limiter circuit 33 and is frequency demodulated by an FM demodulator 34. Then the demodulated signal is fed to a de-emphasis circuit 35 to produce a video signal.
Generally, a normal video signal, as depicted by a waveform shown in FIG. 19b, which varies from a black level to a white level, produces a picture on a CRT as depicted in FIG. 19d. An abnormal video signal, as depicted by a waveform shown in FIG. 19a distorted by so-called "inversion phenomenon," will cause a picture as depicted by a illustration of FIG. 19c. This is explained as follows.
In the video storage/reproduction apparatuses, the lower side band of the FM signal is emphasized due to electromagnetic transducing characteristics. The limiter circuit 33 in FIG. 15 functions to detect the zero crossing points of the signal therethrough. If an absence of the zero crossing points exists in the signal, then an abnormal FM signal will appear at the output of the limiter circuit 33. When the limiter output is demodulated, an abnormal video signal having an abrupt drop in level will be produced as depicted by the waveform in FIG. 19a.
One prior art inversion phenomenon preventing circuit for preventing the inversion phenomenon is a circuit called DL-FM method described on p 98-100 in "NHK home video techniques" written by Katsuya Yokoyama, published by Nippon Hoso Shuppan Kyokai. FIG. 16 is a block diagram of such an inversion phenomenon preventing circuit according to the DL-FM method. FIG. 18 is a waveform diagram for showing waveforms at various points of the block diagram in FIG. 16.
A normal FM signal (i) in FIG. 18 is an original FM signal to be stored, in which points A to J indicate the time at which the zero crossing points are supposed to exist. The signal (i) is more emphasized at the lower side band thereof than the upper side band thereof, as shown by a signal (ii) due to lower side band emphasis effect encountered when the signal (i) passes through the electromagnetic transducing system. It should be noted that the reproduced FM signal (ii) is missing zero crossing at points E and F. This absence of the zero crossing points will cause the previously described inversion phenomenon. In the mean time, a signal (iii) exiting through a high pass filter, or HPF 36 has been emphasized upper side band thereof, there being no absence of the zero crossing at points E and F. The signal (iii) is amplitude limited by the limiter 37 to thereby produce a signal (iv) in FIG. 18. However, it should be noted that the signal (iv) has its zero crossing points at positions in time axis slightly shifted from those of the original FM signal (i). The shift in zero crossing points is also caused by another source which will be described later. The shift in zero crossing points is a source of deteriorated SN ratio in the video signal when the FM signal is demodulated. The signal (ii) is also input to a low pass filter or LPF 39 and is emphasized its lower side band, resulting in a signal (v) which has a very little shift in zero cross points. The signal (iv) and the signal (v) are aligned by an equalizer 38 so that their phase is equal with respect to each other and are then added by an adding circuit 40, thereby producing a signal (vi). The signal (vi) now has less shift in zero cross points caused by noise as compared to the signal (iv), indicating that SN ratio of the video signal obtained will be better in demodulating the signal (vii) than in demodulating the signal (iv). Having a deteriorated SN ratio thereof by the HPF 36 that suppresses lower frequencies, the signal from the equalizer 38 is added by the adding circuit 40 to the signal (v). Thus adding these two signals places a limitation on the improvement of SN ratio of the reproduced video signal, though the circuit arrangement in FIG. 16 can provide better SN ratio than a circuit arranged only with the HPF 36 and the limiter 37. In other words, if recovery of the missing zero crossing points in the reproduced FM signal is a first consideration, then the SN ratio of the video signal will be poor; conversely if improving the SN ratio is a primary importance then the missing zero crossing points will not be adequately recovered.
The shift in zero crossing points is also caused if the frequency response characteristic of an FM transmission line is not flat. The frequency response of the FM transmission system varies, particularly in the DL-FM method, in accordance with the ratio of the level of the output from the LPF 39 to the level of the signal through the HPF 36 and the limiter circuit 37, thus causing shift in zero crossing points. Such a shift in zero crossing points is a source of changes in the frequency characteristic of the demodulated video signal which in turn causes a distorted video signal. Also, a change in the level ratio of the signals to be added causes a change in effect of suppressing the inversion phenomenon, which in turn causes a change in the frequency response characteristic of the demodulated video signal and distortion thereof.